The present invention relates, generally, to apparatus for and methods of the thermal management of electronic devices (whether die-up, die-down, discrete surface mount technology or discrete through-hole) mounted on printed circuit boards.
Thermal management of electronic devices on printed wiring boards is usually accomplished in one or more of three ways: (1) by providing enhanced conduction paths to heat transfer surfaces which are at lower temperatures than the device; or (2) by increasing convection (natural or forced) from the surface of the device to a lower temperature surrounding ambient fluid, usually air; or (3) radiation. In many applications, especially military avionics, the convection and conduction methods are usually mutually exclusive of each other.
With through-hole microcircuit devices conduction heat sinks usually include thermally conductive metal strips located under the body of the device and above the surface of the printed wiring board. The strips conduct heat from the bottom of such a device to a lower temperature heat transfer device, sometimes referred to as a cold wall. The metal strips are often arranged in a contiguous pattern which allows for fabrication by machining and attachment to the printed wiring board. A cold wall is a heat transfer device, such as a hollow structure through which a cooling medium (e.g., air, water) flows.
With through-hole conduction heat sinks, a unique heat sink is usually created for each printed wiring board. The design and fabrication of the heat sink is therefore dependent upon the precise placement of the components on the printed wiring board. Also, such conduction heat sinks cannot be used with surface mounted devices (e.g., quad-flat packs, leadless chip carriers). Conduction heat sinks are not compatible with die-up components.
Commercially available convection heat sinks for microcircuits generally include extended surfaces (e.g., fins, pins, cones, plates) joined to a mounting surface. The heat sink is usually only attached to the electronic component with a thermally conductive adhesive. Such attachment is sometimes augmented with mechanical fasteners to the printed wiring board itself. With commercially available convection heat sinks, the dynamic load contributed by the mass of the heat sink during vibration must be supported by the solder joints of the device being cooled. This can lead to increased fatigue of the solder joints. If the heat sink is attached to the printed wiring board, holes must be added to the printed wiring board. Such holes, and the associated required keep out areas for mechanical attachment, increases the difficulty of routing electrical traces and component placement. It also reduces the space available for component placement.
In addition, current and future printed wiring assemblies are (or will) require integrating a variety of electronic components on the same assembly, such as: die-down (i.e., military, conduction cooled) components; die-up (i.e., commercial, convection cooled) components; discrete surface mount technology (SMT); and discrete through-hole components. An example of this is integrating commercial components into environments where there is less margin between the temperature rating of the commercial component and the temperature of the cooling medium. A specific example is integrating convection cooled parts into conduction cooled military avionics.
U.S. Pat. No. 5,930,115 to Tracy et al. discloses a thermal management structure which is designed to provide both mechanical isolation and heat removal for an unpackaged semiconductor die mounted directly on a printed circuit board substrate. The thermal management structure sandwiches the unpackaged semiconductor die and printed circuit board substrate between two heat sink pieces and conductively removes heat from the die and the substrate. Either or both of the heat sinks may have expanded surfaces, such as pins, fins or the like, which increase the surface area of the heat sink to enhance passive or forced convective heat removal to the ambient environment. These heat sinks may also be designed to engage other components (of the end product in which such heat sinks are employed, such as the chassis or frame) to conductively remove heat away from both the heat sinks and the unpackaged semiconductors.
It is an object of the present invention to maximize the heat transfer from components, both die-down and die-up in a common assembly, wherever they may be located on a printed wiring board, by utilizing integrated convection and conduction paths (for die-up components) and conduction paths (for die-down components).
It is an object of the present invention to effectively integrate the use of convection cooled commercial components into conduction cooled military avionics electronic assemblies.
It is another object of the present invention to minimize the impact of the thermal management of both die-up and die-down components on the electrical design of the printed wiring boards (e.g., component placement, area available for trace routing, etc.).
It is another object of the present invention to provide for a thermal management system which allows component placement and electrical design to occur independently of heat sink design.
It is yet another object of the present invention to simultaneously provide integrated conduction and convection paths for die-up components.
It is yet another object of the present invention to provide a common frame for a family of printed wiring assemblies, and to provide such frame with multiple heat sink mounting locations.
It is yet another object of the present invention to provide a frame for a printed wiring assembly and to attach heat sinks to such a frame to reduce the dynamic load imparted to the electronic component and lessen fatigue effects on solder joints. The heat sink(s) are attachable to the frame by a variety of methods.
It is yet another object of the invention to provide a thermal management system which does not require holes in the field of the printed wiring board, to thereby eliminate the need for keep out areas, and increase the area available for component placement and routing electrical signals.
It is yet a further object of the present invention to stabilize heat sinks by attaching them to the frame at more than one location.
It is yet another object of the present invention to maximize the heat transfer from components, both die-down and die-up in a common assembly, wherever they are located on a printed wiring board, by utilizing integrated convection and conduction paths (for die-up components) and conduction paths (for die-down components) and to allow access to all, or at least some, of such components for inspection and/or electrical testing.